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Abstract

Cosmic rays are atomic nuclei arriving from outer space that reach the
highest energies observed in nature. Clues to their origin come from studying
the distribution of their arrival directions. Using \(3 \times 10^4\) cosmic rays
above \(8 \times 10^{18}\) electron volts, recorded with the Pierre Auger
Observatory from a total exposure of 76,800 square kilometers steradian year,
we report an anisotropy in the arrival directions. The anisotropy, detected at
more than the 5.2\(\sigma\) level of significance, can be described by a dipole
with an amplitude of \(6.5_{-0.9}^{+1.3}\)% towards right ascension \(\alpha_{d} =
100 \pm 10\) degrees and declination \(\delta_{d} = -24_{-13}^{+12}\) degrees.
That direction indicates an extragalactic origin for these ultra-high energy
particles.

We review the possible mechanisms for the generation of cosmological magnetic fields, discuss their evolution in an expanding Universe filled with the cosmic plasma and provide a critical review of the literature on the subject. We put special emphasis on the prospects for observational tests of the proposed cosmological magnetogenesis scenarios using radio and gamma-ray astronomy and ultra high energy cosmic rays. We argue that primordial magnetic fields are observationally testable. They lead to magnetic fields in the intergalactic medium with magnetic field strength and correlation length in a well defined range. We also state the unsolved questions in this fascinating open problem of cosmology and propose future observations to address them.